I have quit MFT, now Sony FF, but still remember well...
I had EM5II and EM1II and used them with the 12-40mm f/2.8 lens in Zen 170mm dome in Nauticam housings (pretty similar to the 12-45mm). This was a brilliant solution for fish portraits, but for me this would not be wide enough for most of the WA sceneries...
Native solutions would be the Zuiko 8mm fisheye (in 4" or 140mm domport) or, when rectilinear, the Zuiko 8-25mm f/4 in 170mm or 180mm domeports. Even better, for my taste, the Tokina 10-17mm fisheye with Metabones 0.71x adapter or the Canon 8-15mm f/4 with Metabones 1x, both in Zen 100mm or Nauricam 140mm domeports (I really miss a zoom fisheye with the FF system I have now)...
Wolfgang